The present invention relates to a semiconductor laser formed of a III group nitride semiconductor.
FIG. 1 is a schematic cross-sectional view showing a conventional ridge waveguide type III group nitride semiconductor laser. A semiconductor laser of FIG. 1 has a stack structure including a GaN buffer layer 202, an n-GaN contact layer 203, an n-GaN buffer layer 204, an n-AlGaN cladding layer 205, an n-GaN guiding layer 206, an InGaN-MQW active layer 207, a p-AlGaN cap, layer 208, a p-GaN guiding layer 209, a p-AlGaN cladding layer 210, and a p-GaN contact layer 211 on a sapphire substrate 201. Since the sapphire substrate is insulative, a portion of the stack structure is etched down to the n-type contact structure 203 in order to expose a region to which an n-type electrode is attached. Furthermore, a portion of a mesa structure is etched down to the p-type cladding layer 210 in order to form a ridge waveguide. In these processes, a dry etching method is employed, and an SiO2 protect film 214 is added for protecting the etched portion.
FIG. 2 shows a relationship between the thickness of the residual p-cladding layer and an effective refractive index difference between the inside and the outside of a stripe (a ridge portion) (a curved line of the conventional example shown in FIG. 2). In the conventional ridge waveguide type III group nitride semiconductor, by utilizing the refractive index difference caused by the difference in thickness of the p-AlGaN cladding layer 210 between inside and outside the ridge portion as shown in FIG. 2, an effective refractive distribution in a (A) portion and a (B) portion is formed, thereby controlling a transverse mode. The control for the effective refractive index in the (B) portion of FIG. 1 is conducted by regulating a film thickness T of the p-AlGaN cladding layer 210 which has been left unetched.
Thus, optical characteristics wherein a light-emitting angle in the vertical direction is 34xc2x0 and a light-emitting angle in the horizontal direction is 7xc2x0 are obtained under the CW operation at a room temperature. Furthermore, a device duration under the CW operation at a room temperature is about 35 hours. FIG. 3 shows a variation of an operation current of the conventional ridge waveguide type III group nitride semiconductor laser under the CW operation at a room temperature.
However, in the conventional ridge waveguide type III group nitride semiconductor laser as shown in FIG. 1, there was a problem that fabricating a semiconductor laser having a uniform transverse characteristic with a high yield is extremely difficult. Dry etching such as RIE, RIBE or the like is employed for etching because no suitable chemical etchant exists for the III group nitride semiconductor, and the control of film thickness for a Pxe2x80x94AlGaN layer 210 of a portion (B) in FIG. 1 is conducted by time control because no suitable etching stop layer exists. However, time control or else employ a less precise technique. As a result, a film thickness of the Pxe2x80x94AlGaN layer 210 varies between plural lots or in the same wafer, whereby controllability of the transverse mode is considerably damaged, and the production yield deteriorates.
Another problem is short lifetime under the CW condition at a room temperature. The inventor of the present application has discovered that this results from using dry etching as a processing method for forming a stripe-shaped ridge shape. More specifically, the above problem results from side surfaces and a bottom surface of a semiconductor to be etched being damaged by an etching treatment, thereby causing a crystal defect, and pinholes being present in SiO2 of an SiO2 protection film covering a p-AlGaN cladding layer on the side surface of the ridge and outside the ridge, whereby the crystal surface in fact cannot be sufficiently protected.
The present invention is made in light of the above conditions, and an object thereof is to provide a semiconductor laser having a single transverse mode characteristic, which can be fabricated with high production yield.
A compound semiconductor laser of a III group nitride semiconductor according to the present invention includes a first cladding layer of a first conduction type formed on a substrate, an active layer formed on the first cladding layer; a second cladding layer of a second conduction type formed on the active layer; and a buried layer formed on the second cladding layer, the buried layer having an opening portion for constricting a current in a selected region of the active layer, wherein an upper portion of the second cladding layer has a ridge portion, the ridge portion residing in the opening portion of the buried layer, and the buried layer does not substantially absorb light output from the active layer, and the buried layer has a refractive index which is approximately identical with that of the second cladding layer, whereby the above object is achieved.
In one embodiment, a light guiding layer of the second conduction type having a refractive index of a higher value than that of the second cladding layer, a third cladding layer of the second conduction type, and a contact layer of the second conduction type are sequentially formed in this order on the upper portion of the second cladding layer.
In one embodiment, the light guiding layer is made of InGaAlN.
In one embodiment, the buried layer is a dielectric film including at least one or more types of compounds among a group including TiO2, ZrO2, HfO2, CeO2, In2O3, Nd2O3, Sb2O3, SnO2, Ta2O5, and ZnO.
In one embodiment, the buried layer is made of a ZnMgCdSSe compound semiconductor.
In one embodiment, the buried layer is made of a semiconductor whose composition is approximately identical with that of the second cladding layer.
In one embodiment, the buried layer is insulative or of the first conduction type.
In one embodiment, a contact layer of the second conduction type is formed on the upper portion of the second cladding layer.
A compound semiconductor laser of a III group nitride semiconductor according to the present invention includes a first cladding layer of a first conduction type formed on a substrate, an active layer formed on the first cladding layer, a second cladding layer of a second conduction type formed on the active layer, and a reflection layer formed on the second cladding layer, the reflection layer having an opening portion for constricting a current in a selected region of the active layer, wherein a layer of a semiconductor of the second conduction type, whose composition is approximately identical with that of the second cladding layer, is formed in the opening portion of the reflection layer, and the reflection layer has a refractive index of a lower value than that of the second cladding layer, whereby the above object is achieved.
In one embodiment, the reflection layer is made of InGaAlN.
In one embodiment, the reflection layer is insulative or of the first conduction type.
In one embodiment, a third cladding layer of the second conduction type and a contact layer of the second conduction type are formed on the reflection layer.
Hereinafter, the function of the present invention will be described.
The present invention enables to provide a device structure in which a transverse mode does not vary against a variation of the amount of etching, and to efficiently fabricate a ridge waveguide type III group nitride semiconductor laser with uniform characteristics.
Furthermore, a device with significantly improved operating lifetime is realized in which a crystal defect caused by damage generated in an etching process is prevented from propagating to an active layer under the CW operation by providing a dielectric layer with smaller pinholes outside the ridge-shaped stripe formed by etching or a structure in which a semiconductor layer is formed to be thick, thereby substantially burying a ridge-shaped stripe.
Furthermore, a device with significantly improved operating lifetime is also realized in which a crystal defect caused by damage generated in an etching process is prevented from propagating to an active layer under the CW operation or by using a convex-shaped groove portion, which has formed by etching, as a current path or by providing a structure buried with a semiconductor layer.
FIG. 2 shows an effective refractive index difference between the inside and the outside of the stripe in a device structure having no light guide layer when an etched portion is buried with a material having a refractive index identical with that of a p-cladding layer (a line of the present invention in FIG. 2). As shown in FIG. 2, according to the present invention, the effective refractive index difference between the inside and the outside of the stripe is eliminated. On the other hand, a gain difference occurs in a portion right under a ridge in the active layer and a portion right under the buried layer by a current constriction effect of the ridge buried layer; and as a result, the transverse mode is controlled. In the structure, an error tolerance range with respect to an etching depth in an etching process for forming a ridge is wide, a transverse mode controllability becomes stable, and the production yield of a laser device with uniform characteristics therefore improves.
Furthermore, these materials to be buried do not generate heat due to light absorption because they are transparent with respect to a light having an emission wavelength of the laser, or affect the transverse mode of the laser because they have an approximately identical refractive index with that of the second cladding layer; therefore, they are suitable as a buried layer.
Furthermore, a crystal defect caused by damage generated in an etching process is prevented from propagating to an active layer under the CW operation because a ridge-shaped stripe portion formed by etching is buried with the buried layer, whereby a device with significantly improved operating lifetime is realized. Furthermore, when a III group nitride semiconductor of an approximately identical composition with that of the second cladding layer is used as a material for the buried layer, a difference in a lattice constant between the buried layer and another epitaxial layer disappears. Thus, effects which a stress such as a thermal strain give to a device can be avoided.
FIG. 4 shows a relationship between a p-cladding layer residual film thickness and an effective refractive index difference between the inside and the outside of a stripe (a conventional example in FIG. 4), and a relationship between the p-cladding layer residual film thickness and an effective refractive index difference between the inside and the outside of a stripe when an etched portion is buried with a material having the same refractive index as that of the p-cladding layer (present invention in FIG. 4), in a device structure having a light guide layer. By providing a structure in which the light guide layer is included in the ridge portion as described above, an effective refractive index inside the ridge becomes large as shown in FIG. 4, whereby a light distribution region in a lateral direction is concentrated in the center. As a result, a transverse mode control becomes easier than in a ridge waveguide type laser utilizing a gain difference as described above. When the buried layer is not provided, or when a refractive index of the buried layer is different from that of the second cladding layer, a thickness of the second cladding layer outside the ridge (an etching residual film thickness) is related to a value of an effective refractive index outside the ridge, thereby significantly affecting the characteristics of the laser.
However, in the structure of the present invention, a refractive index of a buried layer is identical with that of the second cladding layer; therefore, a thickness of the second cladding layer outside the ridge (an etching residual film thickness) does not affect an effective refractive index outside the ridge. Thus, it is not necessary to finely control an etching depth, and it is only required that the etching depth reaches a light guide layer-second cladding layer interface such that at least the light guide layer is present inside the ridge. Furthermore, even when over-etched, it is only required that the etching bottom surface is present inside the second cladding layer. Thus, an error tolerance range during etching is large, controllability of the transverse mode becomes stable, and the production yield of a laser improves.
According to the present invention, by providing a semiconductor layer having an opening portion, which is of the first conduction type or insulative and exhibits a lower refractive index than that of the second cladding layer, a device current is concentrated in the opening portion, thereby generating a gain distribution in a lateral direction of the laser. Furthermore, an effective refractive index becomes relatively larger in a semiconductor layer in the opening portion than in other portions, and a light distribution region in the lateral direction is concentrated to the center, whereby the transverse mode control, in addition to an effect of the above-described gain distribution, becomes easier.
Furthermore, by employing InGaAlN as a material for a semiconductor layer deposited on the second cladding layer, which exhibits a lower refractive index than that of the second cladding layer, respective III group nitride layers which are sequentially grown on the layer can be epitaxially grown while suppressing the generation of a defect, thereby improving the reliability of the laser.